1171. Atmospheric Influence on Inertia Measurements

Paper

Abstract

Mass property measurements are almost always performed in an ambient atmosphere and have been recognized as subject to this environment. This is particularly significant for spacecraft and spacecraft components, where the properties in a space environment are the required input for control of the mission.
Atmosphere affects weighing and mass center determinations only to a minimal degree. The major area of concern is measurement of moments of inertia and, in some cases, the products of inertia which are derived from spin balancing operations.
The advantage of performing measurements of these parameters in a vacuum environment is obvious, but this is often not possible and is hardly ever convenient The key to deciding the need for vacuum operations lies in estimating the extent of error implicit in atmosphere in measurement, with regard to necessary accuracy.
This paper reviews some experience with moment of inertia measurement and spin balancing in vacuum at the NASA Goddard Space Flight Center. Spin balancing of the CTS (Communications Technology Satellite) and moment of inertia measurements of the solar arrays for the IUE (International Ultraviolet Explorer) satellite are discussed in detail.
Essential conclusions are that moment of inertia results obtained in atmosphere for items of low density and large frontal area for oscillatory motion may be very inaccurate, primarily due to entrapped or entrained air effects, and that spin balance operations may be adversely affected to the extent that measured items are non-symmetrical about the axis of spin.
A method for approximating the atmospheric measurement error for moment of inertia due to air entrapment and entrainment is discussed.